Probing the solar system.
THE PIONEER SPACE PROBES
The United States began its exploration of more distant parts of the Solar System with its Pioneer space probes launched in 1972 and 1973. These craft were designed to survive the passage through the Asteroid Belt and Jupiter's magnetosphere. The Asteroid Belt was relatively simple to pass through since there were many gaps, but the space probes were nearly fried by ions trapped in Jupiter's magnetic field.
Launched in 1972, Pioneer 10 was the first spacecraft to fly by Jupiter in December 1973, passing within 130,000 kilometres of the cloud-covered surface. Twenty-three low-resolution images were returned to Earth showing Jupiter's turbulent atmosphere and the Great Red Spot. Pioneer 10's greatest achievement was the data collected on Jupiter's moons, its strong magnetic field, and interactions with the solar wind.
About a year later, Pioneer 11 flew to within 48,000 km of Jupiter's surface and sent back seventeen images of the planet to ground crews on Earth. The space probe used the strong gravitational field of Jupiter to swing it on a path towards Saturn - a journey that was to take five years. In September 1979, Pioneer 11 passed within 30,000 km of Saturn's surface and it returned 440 images and data about Saturn's moons and its ring system.
Today, Pioneer 10 and 11 are no longer sending back data, but both are still travelling at about 12 km/s and heading in opposite directions away from the Solar System into deep space. Each craft carries a plaque, with a graphic message, to inform anyone out there about the Solar System, the Earth, and the human race.
THE VOYAGER PROBES
Voyager 1 and Voyager 2 were launched by the USA in 1977 on a mission towards Jupiter, the largest planet in the Solar System. These probes flew by Jupiter in March and July of 1979 before proceeding to Saturn. Each Voyager was equipped with high-resolution cameras, three programmable computers and instruments to conduct a range of scientific experiments. Voyager 2 was actually launched sixteen days before Voyager 1 but Voyager 1 took a faster, more direct route to reach Jupiter first.
The Voyager probes made the following discoveries:
* 22 new satellites (3 at Jupiter, 3 at Saturn, 10 at Uranus and 6 at Neptune)
* Active volcanoes on lo
* An atmosphere and geysers on Triton
* Rings around Jupiter and spokes in Saturn's rings
* Auroral zones on Jupiter, Saturn and Neptune
* Large scale storms on Neptune
* Magnetospheres around Uranus and Neptune
These two space probes provided many spectacular close-up views of the four outer planets known as the gas giants. Both space probes are still moving away from Earth at about 16 km/s and still in operation. They are returning data about cosmic rays in outer space and ultraviolet sources among the stars.
Both spacecraft also have adequate electrical power and attitude control propellant to continue operating until around 2025, after which there may not be available electrical power (from radioisotope thermoelectric generators) to support science instrument operation. At that time, science data return and spacecraft operations will cease. Interestingly, Voyager 1 has passed the Pioneer 10 space probe and is now the most distant human-made object in space.
More spacecraft have been sent to the Moon than any other body in the Solar system, simply because it is so close to Earth. Our exploration of the Moon continues today. The most recent missions are collecting new data from their on board high-resolution instruments. The GRAIL (Gravity Recovery and Interior Laboratory) spacecraft for example, has been creating the most accurate gravitational map of the Moon ever made. GRAIL completed its primary mission at the end of May 2012, after operating continuously for 89 days. The twin probes (Ebb and Flow) collected data covering the entire lunar surface three times. The mission was extended to the end of 2012 to study the Moon's gravitational field in even more depth. To do this, the probe's orbiting altitude was lowered to an average of 23 km (this allowed the probe to clear some of the Moon's features by only 8 km).
In December 2012, the two washing-machine sized spacecraft were deliberately impacted into the lunar surface at an unnamed mountain near the moon's North Pole. The following web site contains great short videos of parts of the mission. For further info see http://grail.nasa.gov.
On 24 May 2013, the Messenger probe completed its 2000th orbit of the planet Mercury. During the past few years, its orbital period has been reduced from twelve hours to eight hours, so it will take only eleven months to complete the next 1000 orbits.
Messenger's primary mission ended on March 17, 2012 but its mission has been extended to address new questions raised by the initial observations.
The probe has performed the first global reconnaissance of the geochemistry, geophysics, geohistory, atmosphere, magnetosphere, and plasma environment of Mercury.
For further info see: http://messenger.jhuapl.edu/.
The only probe orbiting Venus at the moment is Venus Express. It was launched by the European Space Agency in November 2005. At closest approach, the probe is about 250 km above the north pole of Venus and 66,000 km above the south pole. The probe continues to monitor the surface and atmosphere of our neighbour. The mission has been extended to December 2014.
The latest measurements obtained with Venus Express have shed new light on the interaction between the solar wind and the second planet from the Sun. For further info see: http://esa.int/venusexpress/
Mars Science Laboratory (MSL) is the latest robotic space probe sent to Mars by NASA on November 26, 2011. The probe successfully landed a rover called Curiosity in Gale Crater on August 6, 2012. The objectives of the Curiosity rover include investigating the possibility of life on Mars (its habitability), studying the climate and geology, and collecting data for any future manned mission to Mars.
Curiosity is about twice as long and five times as heavy as the Spirit and Opportunity Mars exploration rovers, and carries over ten times the mass of scientific instruments. The MSL spacecraft that transported it to Mars successfully carried out a more accurate landing than previous rovers, within a landing ellipse of 7 by 20 km in the Aeolis Palus region of Gale Crater. This location is near the mountain Aeolis Mons (a.k.a. 'Mount Sharp'). It is designed to explore for at least 687 Earth days (1 Martian year) an area of 5 km by 20 km.
As part of its exploration, the probe measured the radiation exposure in the interior of the spacecraft as it travelled to Mars, and it is continuing radiation measurements as it explores the surface of Mars. This data would be important for a future manned mission.
The rover came to rest on the floor of Gale crater that is close to a 5.5km-high mountain. The plan eventually is to take the rover to the base of this mountain where it is expected to find rocks that were laid down billions of years ago in the presence of liquid water. Curiosity will probe these sediments for evidence that past environments on Mars could once have favoured microbial life.
One instrument on the rover has already had a chance to gather some data. This is the Radiation Assessment Detector (RAD). It is endeavouring to characterise the flux of high-energy atomic and subatomic particles reaching Mars from the Sun and distant exploded stars. This radiation would be hazardous to any microbes alive on the planet today, but would also constitute a threat to the health of any future astronauts on the red planet.
For more information on the mission, visit: www.nasa.gov/mars and http://mars.jpl.nasa.gov/msl.
Another Mars probe still in operation is Mars Odyssey - the probe has been sending back data for over ten years using its Thermal Emission Imaging System. Since arrival in 2002, the cameras on Odyssey have circled Mars some 45,000 times and taken more than half a million images at infrared and visible wavelengths.
For more info see: http://mars.jpl.nasa.gov/odyssey.
Mars Express has been orbiting Mars since December 2003 and Mars Reconnaissance Orbiter since 2006. Both probes will continue their missions for a few more years. See: sci.esa.int/marsexpress and mars.jpl.nasa.gov/mro.
NASA's Dawn spacecraft entered orbit around the asteroid Vesta on 16 July 2011 for a one-year exploration. Vesta is the second most massive asteroid after the dwarf planet Ceres and comprises an estimated 9% of the mass of the asteroid belt.
The Dawn probe gave scientists a much closer view of this object. The most prominent of the surface features is an enormous crater 505 kilometres in diameter centred near the south pole. The Dawn science team has named it Rhea Silvia, after the mother of Romulus and Remus and a mythical vestal virgin. Its width is 90% the diameter of Vesta. The floor of this crater is about 13 kilometres deep, and its rim rises 4-12 km above the surrounding terrain. A central peak rises 23 km above the crater floor. It is estimated that the impact responsible excavated about 1% of the volume of Vesta, and it is likely that the Vesta family and V-type asteroids are the products of this collision. Spectroscopic analyses have shown that this crater has penetrated deep through several distinct layers of the crust, and possibly into the mantle, as indicated by spectral signatures of olivine.
After spending a year mapping Vesta, the Dawn space probe is continuing its journey to the largest asteroid Ceres, arriving there in 2015.
For more information see: http://dawn.jpl.nasa.gov.
The Juno probe launched in August 2011 continues on its way to Jupiter. The probe is currently on the opposite side of the Sun from Earth, beyond the orbit of Mars. But it will be returning for an Earth flyby in October 2013. Its primary mission is to collect data about the formation and evolution of Jupiter while orbiting it for over a year.
For further info see: www.nasa.gov/juno/.
The Cassini probe (launched in 1997) reached the planet Saturn in 2004 and is still orbiting the planet. The probe's twelve instruments have returned a daily stream of data from the Saturn system. The mission has been extended until September 2017.
Cassini is currently orbiting Saturn with a 9.6-day period in a plane inclined 61.7 degrees from the planet's equatorial plane. The most recent spacecraft tracking and telemetry data were obtained on May 22, 2013 by the 70-metre diameter Deep Space Network station at Madrid, Spain. Except for some science instrument issues described in previous reports, the spacecraft continues to be in an excellent state of health with all of its subsystems operating normally. Information on the present position and speed of the Cassini spacecraft may be found on the 'Present Position' page at: http://saturn.ipl.nasa.govimission/presentposition/.
For further info see: www.saturn.jpl.nasa.gov.
The New Horizon's probe crossed the orbit of Uranus on March 18, 2011 en route to Pluto and the Kuiper belt. It will pass the orbit of Neptune on 25 August 2014.
The New Horizon's spacecraft is expected to start taking data on Pluto and its five moons, months before the spacecraft arrives at Pluto. As it gets closer to Pluto, the spacecraft will look for ultraviolet emissions from Pluto's atmosphere and make global maps of the planet in green, blue, red and a special wavelength that is sensitive to methane frost on the surface. The probe would also take spectral maps in the near infrared, to help scientists determine the surface composition and temperatures. The probe will approach within 9,600 km of Pluto and about 27,000 km of the moon Charon. For further info see: www.pluto.jhuapl.edu.
ABOUT THE AUTHOR:
John Wilkinson is a science educator and author of three Astronomy books: The Moon in Close-up (Springer, 2010); New Eyes on the Sun (Springer, 2012) and Probing the New Solar System (CSIRO, 2009). For further details, see his web site at http://astroscimac.com.